An image from NASA's Cassini spacecraft shows a vast river system on Saturn's moon Titan. It is the first time images from space have revealed a river system so vast and in such high resolution anywhere other than Earth.The river valley crosses Titan's north polar region and runs into Ligeia Mare, one of the three great seas in the high northern latitudes of Saturn's moon Titan.

The international Cassini space probe, which has been orbiting Saturn and its moons since 2004, has sent detailed new images of a remarkable, Nile-like river valley on Saturn’s moon Titan. The river stretches more than 400 kilometers from its ‘headwaters’ and flows into a large sea.

It is the first time astronomers have seen a river system this large and in such high resolution anywhere beyond Earth, according to the European Space Agency.

Scientists believe the Titan river is filled with liquid because it appears in the high-resolution radar image to be dark along its entire length, indicating a smooth surface.

The river meanders at several points. But Jani Radebaugh, a Cassini radar team associate at Brigham Young University in the United States, believes the overall straightness of the river valley suggests it follows at least one major fault line, as do several other large rivers seen running into the same Titan sea.

“Such faults - fractures in Titan’s bedrock - may not imply plate tectonics, like on Earth," said Radebaugh, "but [they] still lead to the opening of basins and perhaps to the formation of the giant seas themselves.”

Titan is the only other world astronomers know of that has stable liquid on its surface. But instead of the water that drives Earth's hydrocycle, Titan's rivers, lakes, seas and oceans are filled with liquid hydrocarbons such as ethane and methane.

Cassini first confirmed the presence of liquid ethane in a lake called Ontario Lacus, in Titan’s southern hemisphere, during its 2008 flyby of the Saturnian moon.

The Nile-like river valley on Titan revealed in Cassini's newest high-resolution radar images were first observed in grainy photos sent back by the European Space Agency's Huygens probe, a separate craft that launched with Cassini in 1997 and descended to the moon’s surface in 2005.

The Cassini-Huygens mission is a cooperative project of NASA, ESA and ASI, the Italian space agency.